This versatile Active Filter is ideal for use as an active crossover in loudspeaker systems but has lots of other uses as well. It can be configured as a low-pass filter (for driving sub-woofer amplifiers), as a high-pass filter or as a bandpass filter, simply by moving a few on-board jumper links.

By John Clarke

Specifications

Voltage Gain: adjustable from 0-2; typically set at 1

Frequency Response: filter dependent

Filter Attenuation slope: 24dB/octave or 80dB/decade

Total Harmonic Distortion: typically .003% at 1V RMS

Signal-to-Noise Ratio: >100dB with respect to 1V input and 22Hz to 22kHz unweighted

Active filters are used in many analog circuits to tailor the frequency response. For example, an active filter could be used to prevent signals below 20Hz from passing through to the next stage (eg, to an amplifier). In this case, the filter allows the higher audio frequencies to pass through but blocks the sub-audio signals (including DC).

This type of filter is called a “high-pass” (HP) filter. If a HP filter is incorporated into an audio amplifier, it will prevent the woofer in a loudspeaker system from being driven at very low frequencies. In fact, it could be used as a turntable rumble filter to follow a magnetic cartridge preamplifier.

Preventing a loudspeaker from being driven at very low frequencies is important because such frequencies would cause audible distortion in the sound due to excessive cone movement. In addition, excessive cone movement at or below the loudspeaker’s resonance frequency could damage the loudspeaker.

Similarly, an active filter could also be used to limit signals above 20kHz. This will prevent supersonic signals from driving the loudspeaker and protect the tweeter(s) from damage. This type of filter is called a low-pass (LP) filter; it allows frequencies below a certain frequency to pass through but blocks higher frequencies.